Polymers, such as polyolefins, and especially polyproylene, are widely used to prepare a number of molded articles, such as medical instruments, and especially syringes. These medical instruments are designed to be packaged in sterile containers, opened, used once, and discarded. Conventional ways of sterilizing them, employing steam or ethylene oxide, for example, have the drawbacks of distorting the article because of heat, or requiring the use of somewhat toxic gases. Widely used now are high energy irradiation processes, in which the packaged instrument is exposed to gamma irradiation, e.g., from a cobalt-60 source, or to electron beams, e.g., from a suitable generator. Unfortunately, sterilizing doses, especially of gamma rays, embrittles most polymers, and this is manifested by a decrease in elongation, noted for example, by breaking when a syringe is buckled and/or bent.
The problem of brittle failure after radiation has been overcome in a number of ways. In one, the polymer is mixed with a so-called "mobilizing additive" which is typically a mineral oil. See, for example, Williams et al., U.S. Pat. No. 4,274,932. This has the drawback that the articles are not stable during long term storage, e.g., in warehouses, as determined, for example, in accelerated oven aging tests. Another approach is to use stabilizing additives, such as hindered phenols, as is described for example in Rekers, U.S. Pat. No. 4,460,445. These turn yellow on exposure to sterilization dosages, and this is undesirable. Thiodipropionic acid esters were used by themselves, for improving the color of radiation stabilized articles made from polyolefins, as described in Kelley et al., U.S. Pat. No. 3,537,967, but when used in combination with a hindered phenol, namely 2,6-di-t-butyl-4-methylphenol, the sterilized article became badly discolored, i.e., yellowish to bright yellow. European Patent Application No. 0-087-294, publsihed Aug. 31, 1983, discloses that the use of heterocyclic hindered amines in polyolefins helps to prevent yellow discoloration, but the addition of mineral oil is beneficial in combination to reduce discoloration and embrittlement during irradiation. In this case, long term thermal oxidative stability is compromised by addition of mineral oil.
Of special interest is the disclosure of European Patent Application No. 0-069-342, published Jan. 12, 1983. This discloses that medical equipment can be made from polymers which contain indicators for sterilization treatments. For example, if polyethylene or polypropylene is mixed with hindered phenols, according to this patent, and then molded and subjected to ionizing radiation from a cobalt-60 source, the color changes from colorless to bright yellow and thus the sterile equipment can be differentiated from equipment that has not been sterilized. The drawback in such a process is primarily that the medical profession has a strong aversion to the use of yellow or bright yellow instruments in a hospital environment. Moreover, yellow coloration in polymers is evidence, almost universally accepted, of polymer degradation, not just as a result of irradiation, but also stemming from adverse exposure to heat, light, chemicals, and the like.
It is contemplated by the present invention to provide an improved composition by using a polypropylene of a special type in conjunction with a phenolic stabilizer which is radiochromic, i.e., one which has a tendency to severely discolor whether under high energy radiation or under the normal course of its free radical reaction in the polymer, and incorporating in said composition a low level of a compound, e.g., a dye or a pigment, which will complement the discoloration species of the phenolic and produce an alternate color in the resin, and to articles molded therefrom, which is entirely aesthetically acceptable, while still maintaining the desirable physical properties and clarity of the polymer during and after irradiation with high doses of energy, and during prolonged storage, e.g., of six (6) months or more, for example, in a warehouse.